MILLENNIA OF MAGMATISM RECORDED IN CRUSTAL XENOLITHS FROM SOUTHWEST GREENLAND

Eruption of aillikite and kimberlite magmas involves rapid ascent of mantle-derived magmas loaded with mantle xenoliths and xenocrysts (>30 vol%). Many such events occur during stages of regional alkaline magmatism and the overall duration and chemical trends of such periods are typically well-constrained. In contrast, the dynamics, petrological history and duration of individual eruptions or dyke intrusions are largely unclear. To address this issue, we performed a petrological and speedometric analysis of well-preserved crustal xenoliths from aillikite dikes at Sisimiut and Sarfartôq alkaline provinces, W Greenland. We focused on the rare and understudied crustal xenoliths, which preserve a rich record of melt injection.

The xenoliths are derived from 25-36 km depth and were transported to the sub-surface within 4 ± 1 h (Fe-in-rutile speedometry), during which they were exposed to the magmatic temperature of 1,015 ± 50 °C (Zr-in-rutile thermometry). The ascent stage was relatively slow compared to average ascent rates of mantle cargo in bonafide kimberlites (4-40 m/s), suggesting a slowing-down of transport at shallow levels. Garnet major-element speedometry shows that at that point the lower crust had already been exposed to a variety of magmas for 700 (Sarfartôq) and 7,100 (Sisimiut) yr. Melt infiltration at Sismiut was initially associated with carbonate- and sulfide-rich melt. Absence of such exotic agent and the overall faster magmatic development at Sarfartôq are tentatively linked to higher decarbonation kinetics in the more depleted SCLM at this location.

This study reveals a so far unrecognized stage of early relatively long-lived magmatism in the development of ultramafic alkaline systems. This stage involved non-steady state interaction between ascending magmas and the immediate SCLM wall-rock, during which the composition of both is modified. The progress and duration of this interaction appears to be controlled by the composition of the SCLM. This would imply that kinetics factors describing this interaction could be used to improve models for the evolution of aillikites and similar magmas.